How do glacial processes contribute to the formation of glacial landforms and landscapes Flashcards
what processes of glacial erosion are there
plucking
crushing
abrasion
fluvio-glacial erosion
where will plucking occur? (Rock)
occur where rocks are well jointed and weakened
what is needed for plucking to occur and how does it get there?
meltwater needs to be present
this occurs due to pressure melting point
how does meltwater contribute to plucking
meltwater penetrates the into the joints and then freezes into the rock
what causes the rock to be plucked/ pulled
the immense pulling force as the ice move
how do glaciers move
by rotational slip
what does plucking do to the landscape
leaves a very jagged landscape
what happens to the plucked material
frozen into the glacial ice
what happens once the material is frozen into the glacier
as the glacier moves downslope the material rubs against the valley sides and floor wearing them away
what is abrasion similar to? (Everyday example)
sandpapering
what will coarse rock debris cause (abrasion)
will scrape the rock surface forming scratches called striations
What does fine rock debris cause (abrasion)
smoothens the rock surface and polishes the rock creating a fine ‘rock flour’
what can striations be used for
can be mapped in relict glacial landscapes to reveal the past ice extent and direction
what is glacial crushing
direct fracturing of weak bedrock by the weight of the bedrock above it
what must be present for glacial crushing to occur
the bedrock must first be weakened by intense freeze thaw or repeated glacial advance and retreat causing dilation
what is dilation
rock fracture due to the removal of overlying glacial ice
what is the typical density of glacial ice
0.9g/cm*3
what is the acceleration due to gravity? (actual figure)
9.81m/s
what is the equation for glacial stress
density of ice x acceleration due to gravity x thickness of the glacier
what is fluvio-glacial erosion? (Definition)
basal ice melting beneath temperate glaciers can produce large volumes of melt water which causes fluvial erosion (normal processes related to water erosion)
what are some fluvial erosional processes
abrasion
hydraulic action
attrition
corrosion
what causes a glacier to deposit material
when velocity is reduced or they become overloaded with debris or when ablation increases
what is the name for material deposited directly underneath the tice
till or boulder clay
how far can till be transported
hundreds of kilometres depending on glacial extent even to areas with a different geology
what is an erratic
a fragment of deposited material that has been carried hundreds of kilometres to an area that may contain different geology
why are erractics important
valuable indicators of the direction of past ice movement as the rock types can be traced to their original source
what is an example of erractics
microgranites from small Scottish island of Ailsa Craig are found in Pembrokeshire so they were transported approximately 400km
what is a moraine
a collective term to describe material carried by a valley glacier and deposited to form a range of landforms
what is deformation
gravity and the pressure of ice in the accumulation zone causes ice crystals to slide over each other in a series of parallel planes in a ‘crumpling’ deformation. This can result in deep crevasses at the surface.
what is lodgement? (Definition of process)
occurs beneath the ice mass when the subglacial debris that was being transported becomes ‘LODGED’
occurs when friction between the subglacial debris and bed becomes grater then the drag of the ice
Where does ablation till come from? (deposition)
debris being dumped as glacier ice melts and thaws can include supraglacial and en-glacial material as well as sub-glacial
what is flow (glacial deposition method)
when there is a high meltwater content casing the glacial debris to creep, slide or flow during deposition
what is lodgement till?
Subglacial sediment (e.g., lodgement till) is material that has been eroded from the underlying rock by the ice, creating more rounded clasts. It has a wide range of grain sizes, including a relatively high proportion of silt and clay.
what is ablation till? (
more angular clasts as they are not ground down
the matrix is of larger sized material ad less compacted
what are the ice contact depositional features
Drumlins
recessional moraines
terminal moraines
lateral moraines
medial moraines
what are the lowland depositional features?
Lodgement till
till plains
ablation till
where are lateral moraines found
found between the glacier and the valley side where the velocity is reduced
what is a medial moraine
caused when two different glacier come into contact and lose velocity depositing material leaves a dark line down the centre of the glacier/ glaciers
what is a terminal moraine
found beyond the glacier snout where the material has been deposited due too ablation
what is a drumlin
smooth elongated mounds of till with a long axis parallel to the direction of ice movement
steep stoss gentle lee
what is the equation for elongation ratio
length/width
what are the typical dimensions of a drumlin
10-50m high and 200-2000m long
where are drumlins typically found
in lowland areas
what is the name of the topography with multiple drumlins
basket of eggs topography
what is Menzies theory
1979
drumlins are formed when glacial sediments get deposited when a glacier gets overloaded with sediment
what is the Shaw theory (glacier)
drumlins show evidence of sorted fluvioglacial sediment indicating meltwater formation
how does the elongation ratio indicate glacial speed
the greater the elongation the faster the ice movement
what can glacial landforms in relict areas be used for
studied to reconstruct former ice extent, direction and rate of ice movement and origin
What three areas can meltwater be found in/on a glacier?
Supraglacial
Englacial
Subglacial
Where is meltwater found if its in the supraglacial environment
On top of the glacier
Any water that flows along the top of a glacier
What is Englacial melt water?
Any water within the glacier
Water enters by moulins
What is Subglacial meltwater?
Found at the bottom of the glacier
Caused by geothermal heat or pressure
Creates lubrication for the glacier
What is a moulin?
A roughly circular, vertical well/shaft through which water enters the glacier from the surface
How wide are moulins generally?
10 metres
Where are moulins generally found?
Flat area of the glacier where transverse crevasses can be found
How deep can moulins be?
Can reach the bottom of the glacier
Hundreds of meters deep
Or might only be 10-40m where a Englacial stream will be
What does it mean when a meltwater stream is highly dynamic?
Ever changing and highly variable
What is variable when talking about meltwater streams (what changes)?
Their discharge (amount of water)
Amount of material being carried (sediment load)
What happens to meltwater stream discharges in summer?
Discharges will be very high
Why does the discharge of meltwater streams increase in summer?
As melt water arrives from many meltwater streams from all over the glacier
Where does the summer increase in discharge happen (type of glacier)?
Glaciers in temperate zones where temperatures increase in the summer
What happens to meltwater discharge in winter?
Discharge can stop altogether as temperatures might never rise above pressure melting point
What does diurnal variations mean?
Something changes on a day and night basis
How is meltwater discharge affected by the diurnal cycle?
Greatest discharge will be when the greatest daily temperatures are
What is the lag time of diurnal meltwater fluctuations?
It takes time between when the highest temperature is and when the greatest discharge is
What are extreme glacial flood or discharge events called?
Jokulhlaup
What happens to meltwater discharge when temperature rises above 0 degrees?
Large volumes of turbulent meltwater erode the landscape
When does fluvioglacial deposition occur?
This occurs when temperature and volume of meltwater discharge and velocity decreases
Why are fluvioglacial deposits different from glacial deposition?
Different erosion types (abrasion and plucking vs attrition and hydraulic action)
Different transport (carried by glacier or melt water streams)
Different deposition (deposited randomly or when water volume can’t hold as heavy a load)
What is the difference in clast shape between glacial and fluvioglacial?
Glacial=frozen limiting their movement and maintaining their angular shape
Fluvioglacial= process of attrition in meltwater make clasts more rounded
What is imbrication?
Clasts have a preferred orientation and dip caused by a strong current
What is the difference in imbrication between glacial and fluvioglacial?
Glacial= clasts are aligned int he direction of ice movement but usually horizontal rather than dipping unless part of a push moraine.
Fluvioglacial= aligned in the direction of flow and often dip upstream
What is stratification and grading?
Deposit is layered with coarse sediment at the base grading upwards into progressively finer ones
How stratified is glacial till?
Unstratified clasts are dumped chaotically by glacier
What is stratification and grading like with fluvioglacial debris?
Stratified and graded by seasonal variation in meltwater discharge
Fine grain deposited in spring/summer when discharge is high and a darker coarse layer when discharge falls in autumn/ winter
Is glacial till sorted?
No ice has enough energy to transport a wide range of grain sizes from fine rock flour to boulders
Is fluvioglacial debris sorted?
Seasonal variation in stream discharge sorts the grains into layers of consistent size
What is an out wash plain?
Is a large area of flat land formed of glacial sediments that is found at the snout of the glacier
How does a outwash plain form?
Material is picked from further up the valley and carried by melt water streams
With the coarsest material found closest to the glacier and finer material further away
Deposits are layered based on seasonal variation
Why is fine material carried further?
As the water loses energy the further is travels so finer material require less energy
What are braided meltwater streams?
Are streams that criss-cross across the outwash plain
How do braided meltwater streams form?
Happens when channels become choked with coarse material becuase of seasonal variations in discharge
This makes the water find the most efficient way trough criss-crossing across the plain
What are varves?
Layers of sediment found int he bottom of lakes that are on the fringes of the glacier (front)
Each year will have two lines of sediment light and dark for summer and winter melt streams
How do varves form?
Coarser lighter layers deposited in spring and summer due to the river reaching it peak/ maximum load
Thinner darker layers of finer silts deposited during autumn when discharge decreases
What are varves a good indicator of?
The age of lake sediments as they show the passing of the years
What are kettle holes?
Basins found on the outwash plain mostly circular in shape and filled with water this is called a kettle
How do kettle holes form?
Formed when blocks of ice washed onto the plain away form the glacier, they sink into the sediment melt and leave a gap
These holes then fill with water to form kettles aquatic plants form which over time produces a marshy area
What are eskers?
Long ridges of material running in the direction of ice advance
Features of eskers?
Winding form 5-20m high
Sorted, coarse material usually sands and gravels
Often layered
How to eskers form?
Believed they are formed by deposits of Subglacial streams
High hydrostatic pressure due to the walls surrounding it enabling a large loa to be carried and mean it can flow up small hills
What is a Kame?
moundlike hill of poorly sorted drift, mostly sand and gravel, deposited at or near the terminus of a glacier
How do Kames form?
Material is deposited along the front of a slowly melting or stationary glacier
The sediment builds up in mounds as the ice melts and more sediment is deposited on top of the old debris
They will often collapse when no longer supported by the ice
How do Kames help in kettle hole formation?
They often bury the blocks of ice creating a kame and kettle topography
How do are terraces form?
Form along the sides of the glacier
Summer ice nearest the rock melts forming a depression that allows melt water to flow and deposits material building up the terrace on the valley side
How are Kame terraces identified against lateral moraines?
Kame terraces are sorted due to being transported by water
What can examining sediment size and shape tell us?
Identify past glacial processes
Till or fluvioglacial debris
How is sediment size determined?
Measuring the three axis’s
What is the equation for the Cailleux index?
A+B
——-
2C
What does a Cailleux index of 1 mean?
The clast is a perfect sphere
What does a higher number on the Cailleux index mean?
Its flatter and more angular like ablation till
